The invention relates to a method for producing a Sn based alloy comprising a metal matrix of a metal matrix material, wherein the metal matrix material comprises Sn, and inclusions of a compound material, further referred to as compound inclusions, wherein the compound material contains one element or a combination of elements of the group Ti, V, Zr, Hf, further referred to as dopant, and one or a plurality of other elements, in particular Sn, Cu and/or Nb. Such a method is, for example, known from U.S. Pat. No. 6,548,187 B2, see Ref. [11].
More specifically, this invention relates to the fabrication of Nb3Sn superconductors used to wind magnet coils capable of generating high magnetic fields. Such magnets are used, for example, in nuclear magnetic resonance, particle accelerators and colliders, nuclear fusion devices, and in research of magnetic and electronic properties of materials.
Nb3Sn superconducting wires are produced by variants of one of three main methods: the bronze process, the internal Sn process or the powder in tube (PIT) process. In all these processes, the brittle Nb3Sn superconducting phase is formed by reacting essentially Nb and Sn in the presence of Cu at 600-750° C., for a suitable amount of time. Nb and Sn are present in deformable form in the unreacted state of the wire so that a wire can be manufactured by metallurgical deformation processes like extrusion and wire drawing.
The principal distinction between these processes comes from the way in which the Sn (and Cu) are present in the unreacted ductile wire.
In bronze route wires, the Sn is alloyed with Cu, whereas in powder in tube processed wires a mixture of NbSn2 and Sn powders is the Sn source.
In wires fabricated by the internal Sn process, a pure Sn or a Sn alloy core is surrounded by an assembly of Cu and Nb. In both cases, the Nb is not in direct contact with the Sn, but a Cu layer separates them either as matrix for a multitude of Nb filaments as it is the case for the rod type or as a continuous layer between the Sn core and a Nb tube in the case of the tube type internal Sn wires. A number of subelements as described above are contained in the superconducting wire, encompassed by a Cu matrix.
It is well known that adding certain chemical elements to Nb3Sn leads to an enhancement of the critical current density at high magnetic fields [1]. Among these dopant elements are Ti, Ta, Mo, V, Hf, Zr, Ga, In and Ge. To be introduced in the Nb3Sn filaments they are usually alloyed with one of the constituents of the wire. During the reaction heat treatment they diffuse from their source into the growing Nb3Sn layer. When the composition of these dopant elements in their source wire-constituent is properly chosen, at the end of the reaction heat treatment the formed Nb3Sn material will have the optimal concentration, which leads to the highest critical current density in the wire.
In the particular case of Ti, for example, several methods are currently in use or have been attempted in the past. One of the first techniques used was to lightly alloy the Nb with Ti and use this alloy for the filament material [2-4], the principal disadvantage being the increased cost of the filament material. For bronze route wires the additional elements that enhance the upper critical field were also alloyed with the bronze (to form a Cu—Sn—X alloy, where X is one of the aforementioned elements) [2, 5, 6]. Films or sheets of the additional elements on the surface of the Nb filaments have also been proposed [7] but this makes the manufacturing more delicate and expensive.
In the case of internal Sn wires, Cu—X alloys (where X is one of the elements mentioned above) have also been used instead of the pure Cu that separates the Nb alloy filaments/tubes from each other and/or the Sn core [4, 8]. Ti bearing alloy rods (usually Nb—Ti) can be inserted either in the center of each filament for rod type internal Sn wires [9] or in the wall of the tube for tube type wires before extruding them to form the precursor material. Alternatively, for rod type wires, Ti bearing alloy rods can be stacked with the Nb filament rods in during the assembly of the billet for the production of the precursor [10].
For most of the dopant elements, one of the most convenient ways is to alloy them with the Sn [3, 4, 11]. In certain cases this has the additional advantage that the Sn alloyed with these elements becomes harder, making it more compatible with the rest of the precursor materials from the point of view of mechanical deformation to manufacture the wire.
Cast Sn—Ti alloys are currently used, but in most cases these mold cast alloys have hard precipitates that are relatively large in size (25-100 μm) and interfere with the Nb filaments/tubes of the wire when the diameter of the Sn alloy cores becomes comparable with the size of the precipitates. This interference causes a distortion and even cutting of the filaments or tubes [12], which has a negative influence on the current carrying capability of the final Nb3Sn superconducting wire. It can also cause, during the reaction heat treatment to form Nb3Sn, a leak of the Sn in the Cu that encompasses the subelements of the wire, which has a negative influence on the ability of this Cu to help in removing electrical and/or thermal disturbances in the wire.
In the process of preparing cast Sn based alloys, a homogeneous melt of Sn containing the dopant(s) is first prepared by dissolving the dopant(s) in the liquid Sn at a suitable temperature. Subsequently the melt is cast in a mold to form a solid billet. Because of the particular equilibrium between Sn and the dopant element(s), compound precipitates containing Sn and doping element(s) form during the cooling of the liquid, before the solidification of the Sn Nagai et al. have proposed to cast the melt in a Cu mold in order to accelerate the cooling of the melt, thus obtaining a Sn based alloy with finer Sn—Ti compound precipitates [11]. However, the cooling rates for larger material quantities that need to be prepared for practical applications are severely limited in this approach.
The situation is similar for the Sn—Hf, Sn—V and Sn—Zr systems, with large liquidus temperature and low solidus temperature at the compositions of interest (up to 10 atomic % dopant), which leads to difficulties in preparing alloys with small precipitate size.
It is also known to produce Ti-based alloys with a Sn content of 2-12 weight % from elemental Ti and elemental Sn powders, see [16].
A way of preparing solid metal from a melt at fast cooling rates is spray forming, also known as Osprey process. In such a process, as described by several patents of Osprey Metals Ltd. (United Kingdom) [13, 14], a stream of molten metal is broken by a high pressure gas atomizer into fine droplets that form a directed jet. The spray of droplets is collected by a moving or rotating table onto which the material accumulates to form a dense, solid body that can be later deformed or machined to the desired shape and dimensions.
It is the object of the invention to introduce a method for preparing a Sn based alloy containing finer compound inclusion with a dopant in order to produce Nb3Sn superconductor material with a superior current carrying capacity.